An electrophotographic method is as described below. A photoconductive substance is utilized so that an electric latent image is formed on an image bearing member (photosensitive member) with various means. Next, a toner image is formed by developing the latent image with toner, and the toner image formed with the toner is transferred onto a transfer material such as paper as required. After that, the toner image is fixed, onto the transfer material with heat and pressure, whereby a recording medium is obtained.
Properties requested of an electrophotographic apparatus have become more and more sophisticated in recent years, and examples of the properties include:    (1) an increase in speed at which the apparatus outputs an image;    (2) an improvement in image quality to respond to a request for a high-resolution, high-definition image;    (3) stability with which high image quality can be prevented from being impaired over a long time period;    (4) high color reproducibility; and    (5) the achievement of energy savings such as a lower power consumption.
A high-productivity electrophotographic apparatus has been attracting attention in recent years because of its potential to supersede an offset printing apparatus. High levels of techniques are requested of the high-productivity electrophotographic apparatus which outputs high-quality color images stably at a high speed. In view of the foregoing, the improvement of an image processing portion, the improvement of an electrophotographic process, and the improvement of a material for the apparatus have been continued; the improvement of toner with which an image is formed, is also important.
Toner based on a pulverization method excellent in low-temperature fixability has been conventionally developed in a vigorous fashion and marketed as toner for high-productivity electrophotographic apparatuses. However, the toner based on a pulverization method involves the following problem: a resin to be used in the toner must be selected from resins each excel lent in heat-resistant storage stability, so the number of resin alternatives is small, and a drastic improvement in low-temperature fixability of the toner is hardly achieved. Further, the toner involves the following problem: upon sharpening of the particle size distribution of the toner for the acquisition of high developing performance, the yield in which the toner is produced reduces, or an additionally large number of production steps for the toner are needed.
In addition, a particle of the pulverized toner is of an amorphous shape, so the toner may be additionally pulverized by stirring or a contact stress in a developing device when the toner is used in a high-speed, high-productivity apparatus. As a result, the following situation may arise: the generation of a fine powder of the order of submicrons, the exposure of a wax, and the embedment of a flowability-imparting agent in the surface of the toner occur, so the quality of an image formed with the toner reduces.
Meanwhile, a reduction in particle diameter of toner has been advanced with a view to improving resolution and definition, and, at the same time, spherical toner has started to be suitably used with a view to improving transfer efficiency and flowability.
In addition, a wet method has started to be preferably employed as a method of efficiently preparing spherical toner particles each having a small particle diameter.
A conventional wet method has been a method of preparing toner particles on the basis of a polymerization method such as a suspension polymerization method or an emulsion polymerization method. Meanwhile, one conventionally known effective method is the following approach: the sharp melt property of the binder resin of toner is improved so that an image formed with the toner can be fixed at an additionally low temperature. However, each of the above polymerization methods involves the following problem: the binder resin of the toner is limited to a vinyl resin.
In view of the foregoing, JP 2004-198692 A and JP 2002-169336 A each propose, as a wet method, a dissolution suspension method involving: dissolving a resin component in an organic solvent immiscible with water; and dispersing the solution in an aqueous phase to form oil droplets so that spherical toner particles are produced. The approach can easily provide spherical toner particles each using a polyester resin excellent in low-temperature fixability as a binder resin and each having a small particle diameter. However, the method may involve the emergence of a problem similar to that in the case of such pulverized toner as described above because the surface layer of each toner particle is apt to peel from the toner base particle of the particle so as to serve as a fine powder.
JP 2004-354706 A discloses a toner using a polyester resin having a relatively low softening point as a core and a vinyl resin having a high softening point relative to that of the core as a shell. When a capsule type toner the core and shell of which are formed of different materials as described above is used in a high-productivity apparatus, the following problem is apt to occur: a surface layer (B) is apt to peel from a toner base particle (A), and the surface layer serves as a fine powder to reduce the durable stability of the toner.
JP 2006-206848 A discloses a core-shell type resin particle excellent in charging characteristic, heat-resistant storage stability, and heat characteristic, and the particle can be used in toner. However, there is still room for investigation on a preferable combination of a core and a shell for the achievement of compatibility between excellent low-temperature fixability and heat-resistant storage stability.